The present invention relates generally to the field of additive manufacturing techniques, and, more particularly, to a new class of processes for rapid prototyping and manufacture of complex, fully dense three-dimensional components.
Complex three-dimensional components can be created by additive manufacturing processes following the following general procedure. Decompose the desired component design into a series of material layers, partitioned into solid and absent portions, to be grown one on top of another, thereby forming the desired component. Note that the individual material layers need not be flat. The only restriction is that they collectively form a foliation of the desired component design.
The growth apparatus comprises elements which carry out the following process steps. Direct an energy source (e.g., a laser beam) onto the growth surface (initially onto a substrate, later onto the previously grown material layer), thereby forming a localized melt-pool (a molten puddle) on the growth surface. A feed material is then fed into and incorporated into the melt-pool (e.g., by melting and alloying with the liquid of the melt-pool, or by dissolving and forming a solution with the liquid of the melt-pool) to provide additional material to the workpiece. The feed material preferably remains solid until actually in contact with the liquid of the melt-pool, differentiating the instant invention from, e.g., laser spray processes in which the feed material is liquefied away from the growth surface, sprayed thereon from a distance in liquid form, and freezes on the growth surface as individual droplets. Introduction of the feed material forces a portion of the original melt-pool to solidify at the melt-pool/growth surface interface, thereby forming a new material layer.
The growth apparatus is rastered along the growth surface to add material to regions of the material layer being grown which are to be solid in the final component design. The rastering process may require full 6-axis motion and control of the growth apparatus to properly manufacture complex components. Continue until the material layer being grown is complete. Repeat the layer growth process until all material layers are grown and the desired component is completed. Given appropriate process conditions, the resulting component can have material properties equal to or exceeding those of the best bulk alloys. Depending on the ultimate application, the component may be ready for immediate use once the additive process is finished, or may require additional finishing steps (e.g., polishing, fine machining, etc.). The above technique, and variants thereof which are clear to one skilled in the art, shall herein be generically referred to as additive manufacturing.
In addition to component manufacture, this general procedure can be adapted to form a single or multiple surface layer on a prefabricated component, an internal portion of a prefabricated component, to welding discrete pieces of a component together, or to forming a billet from which a component will later be machined. The energy source which forms the melt-pool typically comprises a laser beam, but may comprise, with appropriate adaptations, an electron beam, an ion beam, a cluster beam, a plasma jet, an electrical arc, or any other suitably concentrated and intense form of energy source.
There are two primary approaches extant to feeding the filler material into the melt-pool to grown a new layer. First is a powder delivery system, where a fine powder is delivered to the work surface, typically by an inert atmosphere carrier jet. Such systems typically incorporate less than 20% of the powder into the melt-pool, some of the remainder building up on the deposition apparatus and the work surface and some escaping into the local environment. This build up of powder causes difficulty in achieving the desired surface finish, and eventually interferes with the basic function of the laser deposition system. Escape of fine powders into the general environment can present a safety hazard, from the point of view of both health concerns and fire safety. There are stringent OSHA regulations which must be followed to use fine powders in a general manufacturing environment. The special accommodations which are needed to safely operate a powder-based layerwise laser deposition system limit practical application thereof.
The second technique to introduce the filler material into the melt-pool is wire feed, in which a thin wire (wire diameter typically 10-50% of the melt-pool diameter) is fed into the melt-pool. It melts therein and supplies the required growth material for the new layer. The rate of layer growth is primarily related to the spatial extent of the melt-pool, the velocity with which the growth apparatus is rastered along the growth surface, and the rate at with the feed wire is fed into the melt-pool. (Note that this differs intrinsically from systems in which powders, wires, or laminations are arranged in the pattern of the desired layer and welded or sintered to the growth surface.) The rate of deposition can be significantly larger than those generally obtainable using laser powder deposition techniques with the same processing conditions.
FIG. 1 illustrates a typical system as described in U.S. Pat. No. 4,323,756. Here a single laser 10 is focused substantially along a normal onto a growth surface 11, thereby creates a melt-pool 12. A feed wire 13 is directed into melt-pool 12 either from the leading edge or into the side of the melt-pool. (The orientation of the melt-pool is relative to the direction in which the laser beam is rastered along the growth surface.)
The above technique is adequate for simple bi-directional motions of the melt-pool, such as may be encountered in simple laser cladding. To form a complex three-dimensional component in the above manner, however, requires the ability to feed the wire into the melt-pool at any arbitrary radial angle. When fabricating a three-dimensional component using an apparatus having a fixed direction of wire feed, the wire will at times be fed into the true trailing edge of the melt-pool. In this worst-case configuration, the pool/wire interface becomes unstable, and the wire will freeze to the growth surface, stopping the growth process and ruining the component. Beyond this obvious difficulty, such systems tend to produce material layers having large and substantially uncontrolled variations in thickness.
There is therefore a need for a layerwise wire deposition manufacturing process which feeds a feed wire into a laser-generated melt-pool in a manner compatible with true three-dimensional additive growth and fabrication. The process must allow practical growth rates (&gt;1 cc/hr) when used for additive manufacturing. Such a process will enable rapid prototyping and manufacture of prototype assemblies, small quantities of components, and precision molds, and will also be useful for application of surface coatings, repairing mechanical components, general laser welding, and manufacture of special materials.